Cheap AC30 build with Antek Toriodal PT

[billyoung]

i dont think price is issue ...think these are bargain. I got the 100va 350v ct

i was impressed with size and weight of it was a great deal also has the sheield

wire. I have not used it as still gathering parts (from all over world) it will be

way overkill for what i am doing. A baby plexi with mini pentodes in class ab

just like a full sized

even getting mustards and lemco caps. Screw the carbon comps I think

just noise sources waiting to happen I think. Most things vintage you cant

f**ckin hear it ........can you hear cotton wire ? I am using silver plated

teflon, it wont melt if iron brushes it and solders beautifully sh*t wire is

awful. Most who chase vintage cant tell difference. I have sound samples of

a JCM800

versus a LND150 mosfet accurately modeled, by a russian guy . The

Frequency response charts were not labelled either (you cant see difference

either). The guy didnt label samples,only one person could discern the

correct one and it wasnt me .......course (should post the zip file)

all had to listen through computers so.................

[VacuumVoodoo]

So why not take the plunge and use a toroidal OT as well?

[potatofarmer]

In case it isn't obvious, Antek rates all their PTs for their total current draw, so subtract the VA you need for your heaters and then you'll see what it can deliver for your B+. Then again, they're also rated for 50 Hz, so you can squeeze a little more current out of them at 60 Hz.

I did a similar build recently with the 1T250 with a quad of 6V6s and a rocket-esque plan; came out great. B+ is solid at around 320V, IIRC. Definitely going to use their iron again.

Edit: this is wrong, the VA rating applies only to the high-voltage taps. Apologies for being a doofus.

So why not take the plunge and use a toroidal OT as well?

Got a source selling them for $40?

[stephenl]

If your interested in toroid OT's, try these guys http://primroseaudio.com/toroidal-trans ... nsformers/
They make KOC's London Power transformers.

[Roe]

Yes, what is the choke resistance?
The higher the better really, within reason, to keep the screen grids happy.
If it's down ~ 100 ohms, consider using a series resistor to make it up to at least 500 ohms.

yes the choke dc resistance should be at least 420 ohms if you use 100R individual screen resistors. I prefer a 470R shared resistor and 100R individual resistors (and a CLC/Pi filter and ss rectifier)

[David Root]

Nice looking build! Some years ago I played around with toroidal PTs and OTs from Trafomatic in Belgrade.

Two things FWIW from my experience:

1) Unless you mount them vertically like KOC does (did?) they take up a lot of chassis real estate. I tried mounting vertically and simply did not think it would physically hold up in professional use. Horizontal mount is safer IMHO.

2) Toroid PTs have inrush current at startup about twice that of an EI design, so think seriously about putting an inrush current limiter thermistor between the wall AC line input and the mains fuse.

[lord preset]

2) Toroid PTs have inrush current at startup about twice that of an EI design, so think seriously about putting an inrush current limiter thermistor between the wall AC line input and the mains fuse.

Something like this?

http://www.mouser.com/ProductDetail/Amp ... AAAA%3D%3D

[billyoung]

I noticed outputs and choke mounted off axises did you use scope or earphone method ? Axis of flux of torroid should be vertical ? or are
you concerned with signal fron choke to output ? (the small ripple ?)

I have very similiar build going antek on a jtm45 chasis miniwatt tho

bill

[billyoung]

why would inrush be such a big concern ???

think will blow breakers or fuses ?

slow blows ??

[lord preset]

I noticed outputs and choke mounted off axises did you use scope or earphone method ? Axis of flux of torroid should be vertical ? or are
you concerned with signal fron choke to output ? (the small ripple ?)

I have very similiar build going antek on a jtm45 chasis miniwatt tho

bill

The OT was positioned with the headphone method and the choke is off axis for mounting convenience.

[billyoung]

given the magnetic field form of a torroid,seems surprising to me. Could hum be from filaments more? if two el84s in a marshall give 18 or 20 watts, wouldnt this be closer to 40 watts than 30 ?

[lord preset]

I have mounted an off-board 470 ohm 10-watt resistor in series with the screen resistors. Now I have plates at 290 and screens at 285 so I feel better about the power tubes living to a ripe(r) old age. Not sure if I will go with the thermistor suggestion. I'm currently under-fused at 2.5A vs the 3A as per the schematic and it hasn't popped. It's hard for me to evaluate whether this is worth doing.

Clips to follow when I get this in a combo cabinet.

[lord preset]

given the magnetic field form of a torroid,seems surprising to me. Could hum be from filaments more? if two el84s in a marshall give 18 or 20 watts, wouldnt this be closer to 40 watts than 30 ?

On paper I suppose so, but it doesn't feel like more than 30 watts. It's not as loud as my Hayseed 30 which is another AC30 clone made with a big Heyboer AC30 PT/OT set.

[lord preset]

Very doubtful it's pushing 40W at those voltages.

I am surprised, however, that it's noticeably not as loud as the hayseed--that makes me think much more likely a voicing difference (and/or tube quality in one v the other) than a pure power difference.

i mean, 1db is about the least difference we can notice and that translates to what, like a 7-10W difference in output power at that range?

Well I am using unmatched eBay bargain 6P14P tubes in this amp vs matched JJs in the Hayseed. IMO I have never experienced a very strong correlation between wattage and perceived volume in amps of similar wattage. Not all "30 watt" amps are equally loud.

[billyoung]

sounds like a reasonable number. Dont discount the conversion effiency of a good output transformer tho. The difference can be huge, look at marshall 900's no balls at all compared to say an 800.
I am working on a little plexi build with a generic jtm45 chassis, antek power, custom output, real mustard caps , lemco vintage tone caps, a few caps i think have good juju. I also think i found new twist on vvr using a real proper regulator along lines of like 7800 kinda thing. I need to proto it as I bought 20 of em. I no longer have a fine iron so will be a challenge. Hoping i can dial up any voltage i want.

I have few more bells n whistles I want to try as well.

Your build caught my eye with the antek etc. . I think they are inexpensive not cheap tho. Should be superior. I am glad to see inrush current hasnt been an issue, no idea why people get nutty over it ? I think one of the reasons Fender tweed champs are cool is probably a bit undersized power transformer,
which could add to dynamics. My transformer is alot larger and heavier than I thought even tho its 100 va, In industrial control 100 va is tiny lol. this
seems bigger to me. Price was excellent on flea bay, under 35 i think shipped.
hammonds are alot higher.